Electron-image tube apparatus with improved acuteness of the image corners



Nov. 30, 1965 H. GREIN 3,221,209

-IMAGE TUBE ELECT AR US WITH IMPROVED UTENESS T IMAGE CORNERS File ay13, 1965 a a a 4 5 Fig.1 1

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Jnvenfol Attorney United States Patent 3,221,209 ELECTRON-IMAGE TUBEAPPARATUS WITH IMPROVED ACUTENESS OF THE IMAGE CORNERS Hermann Greiner,Darmstadt, Germany, assignor to Fernseh G.m.b.l-l., Darmstadt, GermanyFiled May 13, 1963, Ser. No. 279,879 Claims priority, applicationGermany, May 12, 1962, F 36,788 9 Claims. (Cl. 315-10) This inventionrelates to electron-image tube apparatus in which an electron image istransferred between plane electrodes. The invention has particular,though not exclusive application in connection with television pickuptubes, in relation to which it will be chiefly described.

In modern television cameras employing 3-inch imageorthicon pickup tubesthe resolution has already approximately reached the theoretical valuefor 5 mc./ s. In the central zone of the picture field the resolutionhas in general a value of more than 80% of the black-white interval at500 kc./s. A certain loss of resolution arises, however, at the edges ofthe picture, and in particular at the corners Where it may amount toonly some 50% of the maximum black/white interval. It is thereforedesirable as far as possible to equalize the resolution in the cornersto that at the centre-of the picture.

By careful adjustment of the potential applied to the photocathode of animage-orthicon pickup tube it may be determined that the maximumresolution for the image section of the tube occurs at differentsettings for the centre of the picture and for the margins and corners,where the photocathode requires a different potential applied to it. If,for example, the photocathode voltage for optimum resolution at thecentre of the picture is -460 v., the voltage for optimum resolution inthe corners of the picture of the photocathode potential may be found tobe some -485 v.; in other words, a certain amount of field curvaturearises in the image transfer process. In practical operation precedenceis always given to the centre of the picture, so that the effect of thefield curvature is at its maximum.

The origin of the field curvature mentioned may be understood from thefollowing discussion. Because of the electrode arrangement employed inthe image-converter section of an image-orthicon pickup tube theequipotential lines in the image section do not lie in planar surfaces.In front of the photocathode they are often curved to somewhat of a bellform. As the electrons are withdrawn from the photocathode, owing totheir very low initial velocity they at first describe a path runningexactly perpendicular to the equipotential surfaces. Since the ratio ofthe electric field strength to the magnetic field strength at the edgesof the photocathode is very high in comparison with that at the centre,an electron from the centre of the photocathode will follow arectilinear path to the target, while an electron emitted from an edgeof the photocathode will follow an outwardly curved path. As a firstresult of this the paths of marginal electrons are therefore somewhatlengthened. More important, however, these electrons now cross the linesof force of the magnetic field at an angle which is not negligible, sothat they now follow spirally curved paths resulting in a substantialelongation of the path length. Marginally-emitted photoelectronstherefore form nodes before reaching the target. This effect canapparently only be abolished if it is possible to match the paths of theelectrons and the lines of force of the magnetic field to one another,so that the electrons move as far as possible along the magnetic linesof force and the marginal electrons are prevented from following3,221,209 Patented Nov. 30, 1965 spiral paths. The resultant shorteningof the paths of the marginal electrons ensures that nodes are formed inthe plane of the target by marginal electron bundles as well as by axialbundles.

Since the equipotential surfaces of the electric field are determined bythe arrangement of the electrodes within the image-orthicon tube and, inview of the complexity of the whole system which could only be alteredonly at great expense, the only alternative is to modify the paths ofthe magnetic lines of force in such a manner that the field curvature isequalized.

Focusing systems for the image converters of television camera tubeshave already been proposed in which an additional magnetic field isproduced in the neighbourhood of the photocathode in order to improvethe image forming conditions. In a supericonoscope, for example, inwhich a magnetic field concentrated in the neighbourhood of thephotocathode is employed, a ferromagnetic ring has been arranged withinthe magnetic focusing coil so that its inner margin extended to theneighbourhood of the picture field. The purpose of this arrangement wasto suppress the image aberrations resulting from the S-formed distortiondue to the use of a short magnetic focusing coil. It has also beenattempted to improve the image quality by the inclusion of a winding inthe interior of the magnetic core. In image-orthicon pickup tubes it hasalso been attempted to increase the magnetic field strength in the planeof the photocathode as com pared with that in the plane of the targetelectrode by arranging on the end surface of the focus coil surroundingthe image-orthicon tube a winding of disc form which is traversed by anadjustable current. This arrangement was employed especially with 4/z-inch image-orthicon tubes in order to produce a divergent magneticfield through which the image magnification between photocathode andstorage target was increased. Increase of the resolution in the cornersof the picture was not the purpose of this system and according topresent experience such an effect was not in fact produced. Finally,such an additional winding placed on the end face of the main focusingcoil was used in order to make the magnetic field in the image-convertersection of the pickup tube as homogeneous as possible.

The underlying purpose of the invention, of improving the resolution inthe corners of the picture, is also made difficult because, owing to thealready specified optical conditions in the television camera and thefixed size of the scanning pattern on the target electrode, theconditions under which the image is transferred from the photocathode tothe target electrode can be altered only very little. The use of an ironpole-piece, possible with an additional Winding embedded in it, oran-auxiliary coil applied to the end face of the deflection coils, doesnot therefore lead to the required result, for these measures introducean undesirable modification of the image forming conditions and alsoyield only an inadequate reduction of the loss of resolution in thecorners of the picture.

To solve the problem underlying the invention, of improving the marginalresolution of the electron image projected on to the storage target inan image-orthicon pickup tube, in accordance with the present inventionthere is placed directly upon the faceplate of the pickup tube whichcarries the photocathode an auxiliary coil of small radial height whichis traversed by current in the same direction as that in the mainfocusing coil and is surrounded by this main coil. Preferably thereexists between the outer margin of the auxiliary coil and the innersurface of the main focusing coil a distance which is at least as greatas the radial height of the auxiliary coil winding. This arrangementensures that the magnetic field is modified chiefly in the rearneighbourhood of the auxiliary coil and a marked restriction of themagnetic field occurs in the neighbourhood of the corners of the picturefield on the photocathode, while the field strength at the centre of thephotocathode is relatively little changed.

This arrangement may be still further improved by adding yet anotherauxiliary coil in the interspace between the first-mentioned auxiliarycoil and the main focusing coil, this further auxiliary coil beingtraversed by a current in the direction opposite to that in thefirstmentioned auxiliary coil. The result of this arrangement is that aradial dipole field is produced, which allows a high field-strength tobe generated in the immediate neighbourhood of the edges of the picturefield on the photocathode, this field decaying rapidly towards thecentre of the photocathode. This simple arrangement allows a parabolicfield distribution to be superimposed upon the substantially constantfield strength of the main focusing coil. It has been found that in thecase of the 3-inch irnage-orthicon an increase of the field strength atthe corners of the picture of some 15% as compared with that at thecentre of the picture is necessary in order to eliminate the fieldcurvature. With a voltage on the photocathode suitable for obtainingmaxim-um resolution at the centre of the picture, the central resolutionproduced is then more than 80%, as before, while the marginal resolutionis at least 65%. Variation of the voltage applied to the photocathodeshows that the field curvature is made practically to vanish by the useof the present invention, since it i-s not possible to find aphotocathode voltage at which the corner resolution can be furtherincreased.

The invention will now be more particularly described with reference tothe accompanying drawings, comprising FIGURES 1 and 2, of which:

FIGURE 1 is a schematic representation of a partial longitudinal sectionthrough an image-orthicon pickup tube and its associated focusing coil,such as has been employed hitherto, showing the equipotential surfacesof the electrostatic field and the lines of force of the magneticfocusing field, together with the resultant electron paths, and

FIGURE 2 is a schematic representation of a partial longitudinal sectionthrough a 3-inch image-orthicon pickup tube provided, in accordance withthe invention, with an auxiliary focusing coil of the dimensions foundby experiment to be the most advantageous.

In FIGURE 1, 1 is the envelope of an irnage-orthicon pickup tube whichcarries on its faceplate a photocathode 2. In addition the tube containsa storage target 3 and an accelerating electrode 4, which serves toproduce a suction field for electrons emitted from the photocathode.Other electrodes contained in the tube, which serve to produce ascanning beam and for the collection of the return beam forming thepicture signal are not shown, since the construction of animage-orthicon tube is familiar to the expert and since the presentinvention is not concerned with these portions of the tube.

The tube is surrounded by a long focusing coil 5, the field strength ofwhich is so designed that both the electrons in the image-convertersection formed by the electrodes 2, 3 and 4 and also the electrons inthe scanning beam are focused upon the storage target. The magneticlines of force of the focusing coil 5 are indicated by broken lines a,while the equipotential surfaces of the electrostatic field in front ofthe photocathode are indicated by the broken lines b. Theseequipotential surfaces in front of the photocathode will be seen to besomewhat of bell shape. As mentioned in the introduction, an electronleaving the photocathode at a point near its edge will follow anoutwardly curved path as indicated by dotted line A, while an electronemitted from the centre of the photocathode will travel rectilinearly tothe target as shown by do ted line B. The curvature of A the path of themarginal electron is exaggerated in the drawing for the sake of clarityin illustration. The result of these field conditions in the knownarrangement is that a loss of resolution occurs at the edges of thepicture.

FIGURE 2 shows an arrangement according to the present invention. Thefigure shows to an enlarged scale the image converter section of animage-orthicon pickup tube, the electrodes within the tube being omittedfor the sake of simplicity in illustration. In this embodiment of theinvention there is placed on the faceplate of the envelope 1 whichcarries the photocathode a member 6 in the form of a frame, which isprovided with an auxiliary coil 7. The inner edge of auxiliary coil 7lies closely adjacent to the area of the photocathode occupied by theoptical image which in operation is projected thereupon, and its outeredge has a substantial separation from the inner edge of the mainfocusing coil 5. The frame member 6 is provided at its periphery withseveral projections which engage in corresponding grooves in the mainfocusing coil 5, so that the frame member 6 is firmly anchored to thecoil 5. The magnetic field produced by the coil 7 has the same maindirection as that of the coil 5, and the coil 7 may be so designed thatit is traversed by the same focusing current.

The advantage yielded by the invention is obtained because the imageforming field of the photocathode interacts with that of the coil toproduce a magnetic field with a radial component which gives rise to arestriction of the main magnetic field. Owing to this restriction themagnetic lines of force become curved towards the margins in a mannersimilar to the paths of the electrons shown by dotted line A inFIGURE 1. In this manner it is ensured that, with the magnetic fieldstrength correctly adjusted, the electrons are imaged in the plane ofthe storage target both at the edges and in the centre of the targetelectrode 3, so that the field curvature is substantially abolished. Incertain types of tube, in which it is desirable to vary as little aspossible the image magnification between photocathode and storagetarget, a further improvement may be obtained if the focusing fieldproduced by the auxiliary coil 7 is compensated, as least as regardselectrons emitted at the centre of the photocathode, by a furtherauxiliary coil 8 fitted in the space between the auxiliary coil 7 andthe margin of the frame member 6.

For the former 3-inch image-orthicon cameras there is specified for adistance of mm. between photocathode and storage target, a picturediagonal 10 of some 45 mm. and a raster height 11 on the storage targetof some 35 mm. These conditions are necessary in view of the fact thateven within the image space the magnetic field of the main focusing coildiverges somewhat. Since, however, owing to unavoidable windingtolerances, the lines of force of the coil 5 are often found to be muchtoo divergent, a certain reduction of the divergence is desirable. Thisis likewise produced by the field of the auxiliary coil if thedimensions given below are adopted: Outer diameter of coil 7 =distancebetween photocathode and target Ratio of radial height to width ofwinding section=5:2 Radial height of winding=4 to 5 mm.

The measures described above for improving the corner resolution maynaturally also be employed with advantage in image converter tubes, inwhich there likewise exists the problem of transferring an electronimage between plane surfaces, of which in this case one is aphotocathode and the other is a fluorescent screen lying at a lu'ghpotential or a permeable electron-multiplying membrane.

What is claimed as new and desired to be secured by Letters Patent is:

1. Electron image tube apparatus, comprising an electron image tube,said electron image tube includmg an envelope having a planar member, afirst plane electrode positioned on said planar member in said envelope,a second plane electrode in said envelope, means for productingelectrostatic and electromagnetic fields in said envelope, and means fortransferring an electron image having a central area and peripheralcorner areas from said first plane a first auxiliary coil positionedadjacent the first plane electrode of said electron image tube, saidfirst auxiliary coil being of substantially annular configuration andhaving an inner periphery, an outer periphelectrode to said second planeelectrode under the 5 ery and a radial width between said inner andouter combined effect of said electrostatic and electromagperipheries,the inner periphery of said first auxiliary netic fields; coil lyingadjacent the corners of the area of said an elongated main magneticfocusing coil surrounding first plane electrode occupied by said image,the outer said electron image tube; and periphery of said auxiliary coilbeing spaced from an auxiliary coil positioned immediately adjacent the10 said main focusing coil by a distance at least as great first planeelectrode of said electron image tube, said as the radial width of saidfirst auxiliary coil, said auxiliary coil being of substantially annularconfirst auxiliary coil being adapted to be traversed by figuration andhaving an inner periphery, an outer a current flowing in the samedirection as a current periphery and a radial width between said innerand flowing in said main focusing coil; and outer peripheries, the innerperiphery of said auxsecond auxiliary coil positioned coaxially with andiliary coil lying adjacent the corners of the area of around the outerperiphery of said first auxiliary coil said first plane electrodeoccupied by said image, the within said main focusing coil and adaptedto be outer periphery of said auxiliary coil being spaced traversed by acurrent flowing in a direction opposite from said main focusing coil bya distance at least. that a Current flowing n said ma n focusing coilasgreat as the radial width of said auxiliary coil, Electron image tubeapparatus, comprising said auxiliary coil being adapted to be traversedby a n electron image tube, said tr n image tube incurrent flowing inthe same direction as a current cluding an envelope having a planarmember, a first flowing in said main focusing coil to provide an planeelectrode positioned on said planar member in auxiliary electromagneticfield interacting with the said envelop a second Plalle tr d in saidnelectromagnetic field produced in said electron image v l p ns f rproducing electrostatic and electrotube to reshape the electromagneticfield produced magnetic fields in said envelope, and means for transinsaid electron image tube in a determined manner ferring an electronimage having a central area and in the corner areas of said image tosubstantially peripheral corner areas from said first plane electrodeeliminate field curvature and to substantially avoid to said secondplane electrode under th bin d variation of the electromagnetic fieldproduced in effect of said electrostatic and electromagnetic fields;said electron image tube in the central area of said an elongated mainmagnetic using l surr unding image. said electron image tube; 2.Electron image tube apparatus, comprising a first auxiliary coilpositioned adjacent the first plane an electron image tube, saidelectron image tube inelectrode of said electron image tube, said firstauxcluding an envelope having a planar member, a first iliary coil beingof substantially annular configuraplane electrode positioned on saidplanar member in tion and having an inner periphery, an outer saidenvelope, a second plane electrode in said en- P P Y and a radial WidthhetWeen said invelope, means for producing electrostatic and electronetand outer peripheries, the inner periphery of magnetic fields in saidenvelope, and means for transsaid first ellXiiiary Coil lying adjacentthe Corners ferring an electron image having a central area and of thearea of said first plane electrode occupied by peripheral corner areasfrom said first plane electrode Said image, the Outer periphery of saidfirst auxiliary to said second phase electrode under the combined Coilbeing spaced fmm said main focusing Coil y effect of said electrostaticand electromagnetic fields; a stan t l as as great as the radial Widthof said an elongated main magnetic focusing coil surrounding firstallXiiiafY Coil, said first allXilialY Coil being said electron imagetube; adapted to be traversed by a current flowing in the a firstauxiliary coil positioned adjacent the first plane same direction as aCurrent flowing in said main electrode of said electron image tube, saidfirst auxfocusing coil to Provide an auxiliary electromagnetic iliarycoil being of substantially annular configuration field interacting Withthe electromagnetic field P and having an inner periphery, an outerperiphery duced in said electron image tube to reshape the and a radialwidth between said inner and outer 5O eieetromagnetic field Produced insaid electron image peripheries, the inner periphery of said firstauxiliary tube in a determined manner in the corner areas of coil lyingadjacent the corners of the area of aid fir t said image tosubstantially eliminate field curvature plane electrode occupied by saidimage, the outer and to substantially avoid variation of theelectroperiphery of said first auxiliary coil being spaced magneticfield produced in said electron image tube from said main focusing coilby a distance at least in the central area of said image; and as great athe radial width of aid fi t auxiliary second auxiliary coil positionedcoaxially with and coil; and around the outer periphery of said firstauxiliary coil second auxiliary coil positioned coaxially with andWithin said main focusing Coil and adapted to be around the outerperiphery of said first auxiliary traversed y a current flowing in adirection Opposite coil within said main focusing coil and adapted to bethat of a current flowing in said main focusing coiltraversed by acurrent flowing in a direction opposite Electron image tube ppComprising that of a current flowing in said main focusing coil. anelectron image tube, said electron image tube 3. Electron image tubeapparatus, comprising eluding an envelope having a planar member, afirst an electron image tube, said electron image tube includ- Planeelectrode Positioned on said Planar member ing an envelope having aplanar member, a first in said envelope, a second plane electrode insaid enplane electrode positioned on said planar member in VeiOPe, saidfirst and second Plane electrodes being id envelope, at second plane l td i id spaced a determined distance from each other, means velope, meansfor producing electrostatic and electrofor Producing eiectiostatic andelectromagnetic fields magnetic fields in said envelope, and means fortransin Said envelope, and means for transferring an elecferring anelectron image having a central area and tion image having a Centralarea and Peripheral peripheral corner areas from said first planeelectrode corner areas from Said first Plane electrode to said to idsecond plane electrode under the bi d second plane electrode under thecombined effect of effect of said electrostatic and electromagneticfields; said electrostatic and electromagnetic fields; an elongated mainmagnetic focusing coil surrounding an elongated main magnetic focusingcoil surrounding said electron image tube; said electron image tube; and

an auxiliary coil positioned immediately adjacent the first planeelectrode of said electron image tube, said auxiliary coil being ofsubstantially annular configuration and having an inner periphery, anouter periphery, a radial width between said inner and outer peripheriesand an outer diameter of said outer periphery substantially equal to thedistance between said first and second plane electrodes, the innerperiphery of said auxiliary coil lying adjacent the corners of the areaof said first plane electrode ocner areas from said photocathode to saidraster electrode under the combined effect of said electrostatic havingan inner peripheral area with grooves formed therein;

a frame member positioned adjacent the planar member of the envelope ofsaid image orthicon tube outside the envelope thereof and having anouter peripheral area with projections extending therefrom and adaptedto cooperate with corresponding grooves in said main focusing coil; and

an auxiliary coil positioned in said frame immediately adjacent thephotocathode of said image orthicon cupied by said image, the outerperiphery of said 10 tube, said auxiliary coil being of substantiallyannuauxiliary coil being spaced from said main focusing larconfiguration and having an inner periphery, an coil by a distance atleast as great as the radial width outer periphery and a radial widthbetween said inner of said auxiliary coil, said auxiliary coil being andouter peripheries, the inner periphery of said adapted to be traversedby a current flowing in the auxiliary coil lying adjacent the corners ofthe area same direction as a current flowing in said main of saidphotocathode occupied by said image, the focusing coil to provide anauxiliary electromagnetic outer periphery of said auxiliary coil beingspaced field interacting with the electromagnetic field profrom saidmain focusing coil by a distance at least as duced in said electronimage tube to reshape the great as the radial width of said auxiliarycoil, said electromagnetic field produced in said electron imageauxiliary coil being adapted to be traversed by a tube in a determinedmanner in the corner areas of current flowing in the same direction as acurrent said image to substantially eliminate field curvature flowing insaid main focusing coil to provide an auxand to substantially avoidvariation of the electroiliary electromagnetic field interacting withthe elecmagnetic field produced in said electron image tube tromagneticfield produced in said image orthicon in the central area of said image.tube to reshape the electromagnetic field produced 6. Image orthicontube apparatus, comprising in said image orthicon tube in a determinedmanner an image orthicon tube, said image orthicon tube inin the cornerareas of said image to substantially cluding an envelope having a planarmember, an eliminate field curvature and to substantially avoidilluminated planar photocathode in said envelope variation of theelectromagnetic field produced in positioned on said planar member insaid envelope, 3 said image orthicon tube in the central area of said aplanar raster electrode in said envelope, means for image. producingelectrostatic and electromagnetic fields in 8. Image orthicon tubeapparatus, comprising said envelope, and means for transferring anelectron an image orthicon tube, said image orthicon tube inimage havinga central area and peripheral corner cluding an envelope having a planarmember, an areas from said photocathode to said raster electrodeilluminated planar photocathode in said envelope under the combinedeffect of said electrostatic and positioned on said planar member insaid envelope, a electromagnetic fields; planar raster electrode in saidenvelope, means for an elongated main magnetic focusing coil surroundingproducing electrostatic and electromagnetic fields in said imageorthicon tube; and said envelope, and means for transferring an electronan auxiliary coil positioned immediately adjacent the image having acentral area and peripheral corner photocathode of said image orthicontube, said auxareas from said photocathode to said raster electrodeiliary coil being of substantially annular configuraunder the combinedeffect of said electrostatic and tion and having an inner periphery, anouter peelectromagnetic fields; riphery and a radial width between saidinner and an elongated main magnetic focusing coil surrounding outerperipheries, the inner periphery of said auxsaid image orthicon tube;and iliary coil lying adjacent the corners of the area of an auxiliarycoil positioned immediately adjacent the said photocathode occupied bysaid image, the outer photocathode of said image orthicon tube, saidauxperiphery of said auxiliary coil being spaced from iliary coil beingof substantially annular configurasaid main focusing coil by a distanceat least as great tion and having an inner periphery, an outer periphasthe radial width of said auxiliary coil, said auxcry and a radial widthbetween said inner and outer iliary coil being adapted to be traversedby a curperipheries, the inner periphery of said auxiliary coil rentflowing in the same direction as a current flowing lying adjacent thecorners of the area of said photoin said main focusing coil to providean auxiliary cathode occupied by said image, the outer peripheryelectromagnetic field interacting with the electromagof said auxiliarycoil being spaced from said main netic field produced in said imageorthicon tube to focusing coil by a distance at least as great as thereshape the electromagnetic field produced in said radial width of saidauxiliary coil, the distance beimage orthicon tube in a determinedmanner in the tween the outer periphery of said auxiliary coil andcorner areas of said image to substantially eliminate id main focusingcoil being approximately three fiel c rv re and to n i lly avoidyar'latlon times the radial width of said auxiliary coil, said Of theeleclfomagnetlc field produced In 531d Image auxiliary coil beingadapted to be traversed by a orthicon tube in Central area of Said gcurrent flowing in the same direction as a current Imagfi orthicon Tubepp Comprising flowing in said main focusing coil to provide an an imageorthicon tube, Said image orthicon tube auxiliary electromagnetic fieldinteracting with the cluding an envelope having a Planat member, anelectromagnetic field produced in said image orthiilluminated Planarphotocathode in Said envelope con tube to reshape the electromagneticfield propositioned 011 said Planar member in Said fill/$10139, duced insaid image orthicon tube in a determined a planar raster electrode insaid envelope, means for manner i h Comer areas f i image to P 'Pelectrostatic and electromag fields in stantially eliminate fieldcurvature and to substansaid envelope, and means for transferring anelectially avoid variation of the electromagnetic field tron imagehaving a central area and peripheral corproduced in said image orthicontube in the central area of said image.

. 9. Image orthicon tube apparatus, comprising,

and electromagnetic fields; an image orthicon tube, said image orthicontube inan elongated mam magnetic focusing coil surrounding cluding anenvelope having a planar member an said image orthicon tube, said mainfocusing coil illuminated planar photocathode in said envelopepositioned on said planar member in said envelope, a planar rasterelectrode in said envelope, means for producing electrostatic andelectromagnetic fields in said envelope, and means for transferring anelectron image having a central area and peripheral corner areas fromsaid photocathode to said raster electrode under the combined effect ofsaid electrostatic and electromagnetic fields;

an elongated main magnetic focusing coil surrounding said image orthicontube; and

an auxiliary coil positioned immediately adjacent the photocathode ofsaid image orthicon tube, said auxiliary coil being of substantiallyannular configuration and having an inner periphery, an outer peripheryand a radial width between said inner and outer peripheries, the innerperiphery of said auxiliary coil lying adjacent the corners of the areaof said photocathode occupied by said image, the outer periphery of saidauxiliary coil being spaced from said main focusing coil by a. distanceat least as great as the radial width of said auxiliary coil, thedistance between the outer periphery of said auxiliary coil and saidmain focusing coil being approximately three times the radial width ofsaid auxiliary coil, said auxiliary coil having a cross-sectional areaformed by said radial width and the thickness of said auxiliary coil,said radial width being approximately 2 times said thickness, saidauxiliary coil being adapted to be traversed by a current flowing in thesame direction as a current flowing in said main focusing coil toprovide an auxiliary electromagnetic field interacting with theelectromagnetic field produced in said image orthicon tube to reshapethe electromagnetic field produced in said image orthicon tube in adetermined manner in the corner areas of said image to substantiallyeliminate field curvature and to substantially avoid variation of theelectromagnetic field produced in said image orthicon tube in thecentral area of said image.

References Cited by the Examiner UNITED STATES PATENTS 8/1962 Polinko3l3252 X DAVID G. REDINBAUGH, Primary Examiner.

1. ELECTRON IMAGE TUBE APPARTUS, COMPRISING AN ELECTRON IMAGE TUBE, SAIDELECTRON IMAGE TUBE INCLUDING AN ENVELOPE HAVING A PLANAR MEMBER, AFIRST PLANE ELECTRODE POSITIONED ON SAID PLANAR MEMBER IN SAIE ENVELOPE,A SECOND PLANE ELECTRODE IN SAID ENVELOPE, MEANS FOR PRODUCTINGELECTROSTATIC AND ELECTROMAGNETIC FIELDS IN SAID ENVELOPE, AND MEANS FORTRANSFERRING ANE ELECTRON IMAGE HAVING A CENTRAL AREA AND PERIPHERALCORNER AREAS FROM SAID FIRST PLANE ELECTRODE TO SAID SECOND PLANEELECTRODE UNDER THE COMBINED EFFECT OF SAID ELECTROSTATIC ANDELECTROMAGNETIC FIELDS; AN ELONGATED MAIN MAGNETIC FOCUSING COILSURROUNDING SAID ELECTRON IMAGE TUBE; AND AN AUXILIARY COIL POSITIONEDIMMEDIATELY ADJACENT THE FIRST PLANE ELECTRODE OF SAID ELECTRON IMAGETUBE, SAID AUXILIARY COIL BEING OF SUBSTANTIALLY ANNULAR CONFIGURATIONAND HAVING AN INNER PERIPHERY, AN OUTER PERIPHERY AND A RADIAL WIDTHBETWEEN SAID INNER AND OUTER PERIPHERIES, THE INNER PERIPHERY OF SAIDAUXILIARY COIL LYING ADJACENT THE CORNERS OF THE AREA OF SAID FIRSTPLANE ELECTRODE OCCUPIED BY SAID IMAGE, THE OUTER PERIPHERY OF SAIDAUXILIARY COIL BEING SPACED FROM SAID MAIN FOCUSING COIL BY A DISTANCEAT LEAST AS GREAT AS THE RADIAL WIDTH OF SAID AUXILIARY COIL, SAIDAUXILIARY COIL BEING ADAPTED TO BE TRAVERSED BY A CURRENT FLOWING IN THESAME DIRECTION AS A CURRENT FLOWING IN SAID MAIN FOCUSING COIL TOPROVIDE AN AUXILIARY ELECTROMAGNETIC FIELD INTERACTING WITH THEELECTROMAGNETIC FIELD PRODUCED IN SAID ELECTRON IMAGE TUBE TO RESHAPETHE ELECTROMAGNETIC FIELD PRODUCED IN SAID ELECTRON IMAGE TUBE IN ADETERMINED MANNER IN THE CORNER AREAS OF SAID IMAGE TO SUBSTANTIALLYELIMINATE FIELD CURVATURE AND TO SUBSTANTIALLY AVOID VARIATION OF THEELECTROMAGNETIC FIELD PRODUCED IN SAID ELECTRON IMAGE TUBE IN THECENTRAL AREA OF SAID IMAGE.